Here is how you can target the "Follow us" button on posts in The Columnist Theme in your blog:

#subscribe-blog input[type="submit"] {
background: red;
}

Then you can add CSS rules into that block as needed.

If you're looking to customize that same button for Blogum instead, the same CSS will probably work because I used "#subscribe-blog" which is specific to the form and not actually dependent on the theme's structure. However, if you run into trouble targeting the button on Blogum, just post an example link and details about what you've tried already.